Modulation technique for a limited energy speech transmission system employing phase reversal of alternate pulses

ABSTRACT

A modulation technique for a limited energy speech transmission system (LEST) is disclosed wherein either alternate speech zero crossing pulses or alternate radio frequency pulses are phase reversed prior to transmission.

United States Patent Robert T. Milton Burnt Hills, N.Y.

Dec. 16, 1969 Nov. 30, 1971 General Electric Company Inventor Appl. No.Filed Patented Assignee MODULATION TECHNIQUE FOR A LIMITED ENERGY SPEECHTRANSMISSION SYSTEM EMPLOYING PHASE REVERSAL OF ALTERNATE PULSES 6Claims, 4 Drawing Figs.

US. Cl 332/31 R, 325/138, 325/148, 238/165, 332/44 Int. Cl H03c l/52FieldoISearch 332/I8,31, 44, 23, 37; 328/162-165, 166, 140; 325/42, 65,148, 138

[56] References Cited UNITED STATES PATENTS 3,398,374 8/1968 Kaufman328/165 3,413,412 11/1968 Townsend 328/165 X 3,475,600 10/1969 328/162 X3,486,117 12/1969 332/18 X 3,497,817 2/1970 328/163 PrimaryExaminer-Alfred L. Brody Attorneys-John F. Ahern, Paul A. Frank, JuliusJ.

Zaskalicky, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. FormanABSTRACT: A modulation technique for a limited energy speechtransmission system (LEST) is disclosed wherein either alternate speechzero crossing pulses or alternate radio frequency pulses are phasereversed prior to transmission.

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MODULATION TECHNIQUE FOR A LIMITED ENERGY SPEECH TRANSMISSION SYSTEMEMPLOYING PHASE REVERSAL OF ALTERNATE PULSES This invention relates to aspeech transmission system wherein speech signals are clipped and zerocrossing detected to produce a series of zero crossing pulses. The zerocrossing pulses are then shaped to provide a series of uniform squarewave pulses. The square wave pulses may be filtered to limit theirbandwidth and used to modulate a carrier or they may be used to modulatea carrier directly and the modulated carrier is filtered to limit itsbandwidth. The modulated carrier is then transmitted by any suitablemeans to a receiving station. By transmitting only the zero crossingpulses, considerable savings are obtained in average power output.

At the receiving station, the received signal is detected to obtain theseries of zero crossing pulses. These pulses are then applied to abistable multivibrator which converts the pulses back into the clippedspeech signal. This clipped speech signal, after any desiredamplification and filtering, is used to drive a loudspeaker, whichconveys the intelligence of the original speech signal.

It has been found, however, that the radio frequency bandwidth for thissystem must be about four times the speech bandwidth. If the radiofrequency bandwidth is less than this, zero crossing pulses of thehigher frequency components of the speech tend to overlap. Anyoverlapping of zero crossing pulses makes difficult the detection ofindividual pulses at the receiving station.

In view of the above, it is an object of the present invention toprovide an improved limited energy speech transmission system whereinthe effect of overlapping zero crossing pulses is greatly reduced.

A further object of the present invention is to provide a pulsetransmission system where alternate pulses are reversed in phase toreduce bandwidth requirements.

Another object of the present invention is to provide an improvedlimited energy speech transmission system wherein alternate zerocrossing pulses are reversed in phase, either at the baseband level orat the radio frequency level, prior to transmission.

A further object of the invention is to provide a pulse transmissionsystem wherein the overlapping portions of successive pulses arecombined to produce a null at some point between the pulses.

Yet another object of the invention is to provide an improved modulationtechnique for a limited energy speech transmission system in whichhigher frequency components of the speech can be transmitted.

It is proposed by this invention to improve the basic limited energyspeech transmission system by reversing the phase of alternate zerocrossing pulses, whereby the overlapping portions of the zero crossingpulses will contain a deeper null.

The various features of the invention may be best understood byconsidering the following description and the attached drawings inwhich:

FIG. 1 illustrates a series of waveforms useful in understanding thepresent invention.

FIG. 2 illustrates a specific form of a modified zero crossing detectorused in the present invention.

FIG. 3 illustrates one form of the invention utilizing radio frequencyband pass filtering.

FIG. 4 illustrates another embodiment of the invention utilizingbaseband filtering.

Referring to FIG. 1 there is shown a series of waveforms (a) through (3)illustrating the present invention and its advantages over the basiclimited energy speech transmission (LEST) system disclosed in thecopending application of Roy E. Anderson, Ser. No. 700,310, filed Dec.22, 1967, now U.S. Pat. No. 3,528,011 and assigned to the same assigneeas the present invention. In FIG. la, the waveform 1 represents theinput waveform to the overall system which may, for example, be a speechwave. The zero crossings illustrated in FIG. lamay either be thecrossing of a zero point such as ground or a relative zero point such asthe overall average level of the input speech waveform. FIG. lbillustrates a series of pulses 2 generated in accordance with the zerocrossings of the input waveform. FIG. 1c shows a series of overlappingzero crossing pulses resulting from the zero crossing pulses beingfiltered. The overlap of the pulses 3 is due to the bandwidthrestrictions imposed upon the speech transmission system. In FIG. 1dthere is shown a sum of the overlapping pulses and a threshold level 4used at the receiving station to distinguish one pulse from the next. Ascan be seen from FIG. 1d the relative amplitude of the zero crossingpulses has been reduced by the overlap. Further, it can be seen that ifnoise is introduced into the signal transmitted, it will reduce therelative pulse height even further making it more difiicult todistinguish one pulse from the other.

In FIG. 1e there is shown a series of pulses representing the proposedsolution to the above situation wherein alternate zero crossing pulsesare reversed in phase or inverted before transmission. Here, theoverlapping portions of the successive zero crossing pulses tend tocancel each other out in the overlap region 5 producing thereby a deepernull and a series of pulses having steeper side portions. FIG. 1frepresents the waveforms occurring when the alternate zero crossingpulse phase reversal takes place at the radio frequency level ratherthan at the baseband level. As can be seen from FIG. 1 f, in the overlapregion 5, cycles of the carrier frequency from one pulse tend to cancelout cycles of the carrier frequency from the next pulse due to the phaserelationship between the carrier cycles of succeeding pulses. Thus, theenvelope of the waveform in FIG. If achieves a deeper null in the samemanner as the waveform in FIG. Ie at the baseband level. FIG. lgrepresents the waveform of the detected or full wave rectifiedtransmitted signal. This detected signal has a series of deeper nulls 6that extend further below the threshold level 4 than that shown in FIG.1d. FIG. 1/: represents the series of zero crossing pulses generated atthe receiving station in accordance with the detected signals as shownin FIG. lg. The relative phase shift between FIGS. lb and lg has noeffect on the intelligibility of the reconstructed speech.

Thus, it can be seen from FIG. 1 (a-d) the basic LEST system provides aseries of zero crossing pulses that may tend to overlap due to bandwidthrestrictions. The present invention obviates this by inverting alternatezero crossing pulses prior to transmission so that, at the receivingstation, the individual pulses may be more readily detected by virtue ofthe deeper null occurring between succ ssive zero crossing pulses.

In FIG. 2 there is shown a modified zero crossing detector 10 which canbe used in the preseiit invention to provide alternately phase reversedzero crossing pulses. In the system of FIG. 2 the input speech signal islimited to provide a square wave signal and then differentiated toprovide a series of zero crossing pulses. These zero crossing pulses arethen used to trigger a pair of monostable multivibrators whose outputsare a series of pulses having the same polarity. These series of outputpulses are then applied one to each input of a differential amplifierwhich serves to substract one set of pulses to the other and provides atits output a series of zero crossing pulses in which alternate pulsesare phase reversed or inverted.

Specifically, the modified zero crossing detector I0 comprises a sourceof speech signal 11 connected to the input of a limiter 12 whichprovides at its output a square wave signal representing a clippedversion of the input speech signal. This clipped version of the inputspeech signal is passed through a differentiating means 13 whichprovides a series of pulses at each zero crossing. These pulses arebipolar, that is, there are positive pulses for each positive going zerocrossing and negative pulses for each negative going zero crossing. Thepositive and negative signals are separated into two channels by arectifying means 14 which passes the positive going zero crossing pulsesand rectifying means 17 which passes the negative going zero crossingpulses. The positive going zero crossing pulses are applied to the inputof a one-shot or monostable multivibrator 16 which provides at itsoutput a series of uniform duration pulses in accordance with thepositive going zero crossing pulses at its input. In like manner, theoneshot or monostable multivibrator 19 provides a series of uniformoutput pulses for the negative going zero crossing pulses applied to itsinput. The output pulses from the one-shots l6 and I9 are of the samepolarity, that is, in the example shown in FIG. 2, they are bothpositive going pulses. These pulses from 16 and 19 are applied to theinputs of differential amplifier 20 which serves to subtract the twoseries of pulses from each other, in the well-known manner of operationof differential amplifiers, and provides at its output 21 a series ofzero crossing pulses in which alternate zero crossing pulses arephase-reversed or inverted. This is achieved by virtue of the fact thatone set of pulses for example, the output pulses from element 16 areconnected to the positive input of the differential amplifier 20 and thenegative zero crossing pulses provided by the monostable multivibratorI9 are applied to the negative or subtract input of the differentialamplifier 20. Thus, for each pulse from element 16 the differentialamplifier provides a positive going output pulse. However, for eachpulse from the element 19, the differential amplifier provides anegative going output pulse since this pulse has been applied to thesubtract input of the differential amplifier.

Thus, there has been shown one example of a suitable modified zerocrossing detector for use in the present invention. It should beapparent that modification of the detector may be made. For example, ifthe outputs from the monostable multivibrators l6 and 19 were of unlikepolarity, that is, for example, the output pulses of monostablemultivibrator 16 were positive going and the output pulses of monostablemultivibrator 19 were negative going, the differential amplifier 20could be replaced with a simple summing circuit. Also, it should benoted, in comparison with the basic LEST system, that if the outputs ofthe monostable multivibrators I6 and 19 are of like polarity and areapplied to a summing circuit instead of the differential amplifier 20,the net result is the basic LEST system wherein a series of uniformpulses are produced which are all of the same polarity. Conversely, thebasic LEST system is also provided if the outputs of the monostablemultivibrators 16 and 19 are of unlike polarity and are applied to adifferential amplifier 20. The small waveforms appearing in conjunctionwith the output lines of the various elements in FIG. 2 show the typicaloutputs of the preceding elements.

In FIG. 3 there is shown one embodiment of the overall system employingthe present invention. In this embodiment, the output pulses from themodified zero crossing detector are used to modulate a radio frequencycarrier to provide a radio frequency output signal which is then passedthrough a bandpass filter to achieve the desired bandwidth requirements,and transmitted. The radio frequency signal prior to transmission mayalso be translated to a higher frequency. That is, the modulated radiofrequency may be multiplied by a factor of N, where N is any wholenumber, to achieve a higher frequency output or the modulated carriercan be heterodyned with another source of signal to provide the higherfrequency output.

In FIG. 3 there is shown the modified zero crossing pulse generator 10having its output connected as one input to balance modulator 30. Theother input of balance modulator 30 is connected to the output of aradio frequency source 31. The output of balance modulator 30, which maybe in the form of a double sideband suppressed carrier, is passedthrough a radio frequency band-pass filter 32 to achieve desiredbandwidth requirements which may include conversion to single sideband.Also shown in FIG. 3 is a translating means 33 which can be used asdesired to increase the output frequency of the radio frequency signalfrom the band-pass filter 32.

Filtering at the carrier frequency as shown in FIG. 3 may impose high Qrequirements on the filter when the carrier frequency is very muchhigher than the assigned bandwidth, since the required is proportionalto the ratio of the carrier frequency to the bandwidth. The embodimentof FIG. 4 overcomes this by filtering at baseband. In FIG. 4, the outputsignals from the modified zero crossing detector are first filtered toachieve the desired bandwidth and then used to modulate a radiofrequency source. There is shown in FIG. 4 the modified zero crossingpulse generator having its output connected to low-pass filter 34 whichin turn passes the filtered zero crossing pulses to balance modulator30. The second input to the balanced modulator is connected to radiofrequency source 31. At the output 35 of the balanced modulator 30 thereappears a modulated signal in which alternate pulses are phase reversed.

In connection with FIG. 1, the output at 35 of FIG. 4 is showndiagrammatically in FIG. I as the waveform in FIG. If. In FIG. If anyoverlap between succeeding pulses from the output of the modulator 30would tend to cancel each other out. The output of the low-pass filter34 in FIG. 4 is shown in FIG. I as the waveform of FIG. le. FIG. 1 doesnot directly show the waveforms involved in the operation of the circuitof FIG. 3; however, the output of the modified zero crossing pulsegenerator 10 would correspond to FIG. 1b with alternate pulses reversedin phase or inverted. The output of the bandpass filter 32 in FIG. 3would correspond to FIG. If wherein alternate radio frequency pulses arephase reversed.

While a specific embodiment of the present invention has been shown anddescribed, many modifications will be apparent to those of skill in theart. For example, the balanced modulation of a carrier is not the onlymeans by which the objects and advantages of the present invention canbe achieved. Other forms of modulation may also be used.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An improved limited energy speech transmission system wherein thezero crossings of the speech waveform are transmitted, the improvementcomprising:

zero crossing detector means for sensing the zero crossings of an inputanalog waveform and for generating a pulse train by producing a pulse inresponse to each zero crossing;

means for reversing the polarity of alternate zero crossing pulses toproduce a modified pulse train; and

means for modulating a radio frequency carrier signal with the modifiedpulse train prior to transmission thereof.

2. The system as in claim I wherein said radio frequency modulatingmeans comprises:

a source of the radio frequency signal;

balanced modulator means having coupled as inputs thereto the radiofrequency signal and the modified pulse train; and

a radio frequency filter coupled to receive the output of saidmodulator.

3. A system as in claim I wherein said radio frequency modulating meanscomprises:

a low-pass filter coupled to receive said modified pulse train; a sourceof the radio frequency signal, and balanced modulator means receiving asinputs the output of said low-pass filter and said radio frequencysignal.

4. The system as in claim I wherein said zero crossing means and saidpolarity reversing means detector comprises:

limiter means for limiting the amplitude of the input waveform toproduce square waves;

differentiating means for differentiating the square waves to producepositive and negative spike pulses;

rectifying means for separating said positive and negative spike pulses;

a pair of pulse generating means, one triggered by said positive spikepulses and the other triggered by said negative spike pulses, forproducing output pulses of the same polarity, and

differential amplifier means subtracting the pulses from each pulsegenerator to produce said modified pulse train.

synchronism with said zero crossing pulses; and means for reversing thephase of alternate ones of said radio frequency pulses. 6. A system asdefined in claim 5 further comprising: means for translating said radiofrequency pulses to a higher frequency.

1. An improved limited energy speech transmission system wherein thezero crossings of the speech waveform are transmitted, the improvementcomprising: zero crossing detector means for sensing the zero crossingsof an input analog waveform and for generating a Pulse train byproducing a pulse in response to each zero crossing; means for reversingthe polarity of alternate zero crossing pulses to produce a modifiedpulse train; and means for modulating a radio frequency carrier signalwith the modified pulse train prior to transmission thereof.
 2. Thesystem as in claim 1 wherein said radio frequency modulating meanscomprises: a source of the radio frequency signal; balanced modulatormeans having coupled as inputs thereto the radio frequency signal andthe modified pulse train; and a radio frequency filter coupled toreceive the output of said modulator.
 3. A system as in claim 1 whereinsaid radio frequency modulating means comprises: a low-pass filtercoupled to receive said modified pulse train; a source of the radiofrequency signal, and balanced modulator means receiving as inputs theoutput of said low-pass filter and said radio frequency signal.
 4. Thesystem as in claim 1 wherein said zero crossing means and said polarityreversing means detector comprises: limiter means for limiting theamplitude of the input waveform to produce square waves; differentiatingmeans for differentiating the square waves to produce positive andnegative spike pulses; rectifying means for separating said positive andnegative spike pulses; a pair of pulse generating means, one triggeredby said positive spike pulses and the other triggered by said negativespike pulses, for producing output pulses of the same polarity, anddifferential amplifier means subtracting the pulses from each pulsegenerator to produce said modified pulse train.
 5. An improved limitedenergy speech transmission system wherein the zero crossings of thespeech waveform are transmitted, the improvement comprising: means forsensing the zero crossings of an input waveform and for producing apulse in response to each zero crossing; means for producing a series ofradio frequency pulses in synchronism with said zero crossing pulses;and means for reversing the phase of alternate ones of said radiofrequency pulses.
 6. A system as defined in claim 5 further comprising:means for translating said radio frequency pulses to a higher frequency.